The present invention relates to a charge controlling method and a developer containing a charge-exchange control agent, that is used to make visible electrostatic latent images formed in electrophotography or electrical signals provided in electrostatic recording. More particularly, the present invention relates to a developer that has good characteristics in terms of negative charging, and development and transfer of an image, and to a charge-exchange control agent used therein.
Toner particles contained in developers (toners) may be charged by a variety of techniques such as triboelectrification, contact electrification, irradiation with ions, static induction making use of electrical conductivity, and charge implantation. Of these, the first two methods are most easily available and hence are used extensively. Toners to be charged by either triboelectrification or contact electrification are classified into two types according to the charge imparting mechanism: a two-component developer which is composed of a mixture of toner particles and carrier particles to be charged for the polarity opposite to that of toner charging; and a single-component developer which is charged by the rubbing of toner particles, either with themselves or with charging blades or rollers.
The charging of toner particles in contact electrification or triboelectrification is typically controlled by adding electron donors or acceptors to the toner particles or the charge imparting materials such as carrier particles or blades. Triboelectrification is a complex phenomenon and its mechanism is not fully understood, but in general the electron donor is positively charged and the electron acceptor charged negatively.
Conventionally, the charging of toner particles is controlled by using additives, such as dyes, pigments, surfactants or inorganic powders, in the interior or on the surface of the toner particles, as described, for example, in U.S. Pat. Nos. 3,779,926, 3,893,935, 4,298,672, 3,944,493, 4,007,293, 4,079,014, and 4,394,430, or by using as binder resins those resinous materials having functional groups such as --COOH, --CN, a halogen (e.g., --Cl), --NO.sub.2 or --NH.sub.2, as described, for example, in U.S. Pat. Nos. 3,985,664 and 4,049,477.
However, the use of such conventional additives or binder resins has met with only limited success in controlling toner chargeability, and fully satisfactory levels of stability for storage and relative to environmental factors have not been attained. In particular, color toners allow only limited materials to be used as a charge-exchange control agent since inappropriate materials may cause variation of tone or reduced transparency. Furthermore, dyes or pigments which are used as coloring agents often cause adverse effects on toner chargeability and render it more difficult to control the charging of color toners than in the case of black toners.
It has recently been proposed that finer toner particles be used in order to produce copies with high-quality images as manifested by good reproduction of fine lines or dots in halftone image. If color images are to be formed by the subtractive process using three or four overlapping color toners, the decrease in the size of toner particles has the advantage of allowing a color image of high quality to be readily formed by preventing the excessive buildup of image or the occurrence of excessive gloss on account of overlapping toner layers. On the other hand, because of the small toner particle size, the content of dyes or pigments must be increased in order to achieve coloration that is comparable to that attainable with ordinary sized toner particles and this leads to further difficulties in attempts to successfully control the charging of toners.